1. Field of the Invention
The invention relates generally to symbol clock recovery for a radio communication system and more particularly to a radio system using a global position system (GPS)-based time for synchronizing the transmission and reception of communication symbols.
2. Description of the Prior Art
Communication links commonly communicate with discrete data packets that are separated in time. Such communication links have several benefits as compared to links using continuous communication, however, several issues must be resolved in order for a packet communication link to operate. A first issue is that the receiver must synchronize itself to the times-of-arrival of symbols within the data packet in order to sample the symbols at the correct times. In a conventional packet communication link in order to resolve this issue, a transmitter prepends preamble symbols to the beginning of each data packet. A receiver receives the preamble symbols with a phase lock loop for recovering a symbol clock that is then used for detecting the symbols at the correct times. However, the number of symbols that must be received before achieving symbol synchronization varies depending upon the initial frequency and phase offsets in the symbol clock recovery phase lock loop and the signal-to-noise ratio in a particular application. Therefore, a second issue is that the receiver must synchronize itself to the start of a payload portion of the data packet without knowing how many of the preamble symbols remain when the symbol clock has been recovered. Conventionally, in order to resolve this issue, the transmitter inserts a pre-determined pattern of frame synchronization symbols at the end of the preamble. The receiver uses the frame synchronization symbols for synchronizing itself to the beginning of the payload data symbols. Unfortunately, the preamble and the frame synchronization pattern represent overhead that use up time but do not communicate useful payload information. This overhead is especially disadvantageous when the packets are short. There is a need for a packet communication system where the time overhead of a preamble and a frame synchronization pattern are eliminated.
It is therefore an object of the present invention to provide a communication transmit apparatus including a global positioning system (GPS) receiver for transmitting communication data symbols synchronized to GPS-based time.
Another object of the present invention is to provide a communication receive apparatus including a GPS receiver for using GPS-based time for synchronizing to the communication data symbols.
Briefly, in a preferred embodiment, a communication system of the present invention includes a transmit apparatus for transmitting a communication signal having data symbols synchronized to a pre-determined GPS-based time and a receive apparatus for using the pre-determined GPS-based time for receiving the communication signal and synchronizing to the data symbols. The transmit apparatus includes a transmit station GPS receiver for providing a GPS-based time that is current, a transmit framer for beginning the transmission of a packet of data symbols when the current GPS-based time reaches the pre-determined GPS-based time, and a transmit symbol timer for synchronizing the transmission of data symbols to time ticks of GPS-based time. The receive apparatus includes a GPS receiver for providing the current GPS-based time, a receive framer for receiving the data symbol packet when the current GPS-base time equals the pre-determined time, and a receive symbol timer for using the GPS-based time ticks for aligning to the incoming data symbols. Either the transmit apparatus includes a propagation advance calculator or the receive apparatus includes a propagation delay calculator in order to compensate for the propagation delay of the communication signal between the transmit apparatus and the receive apparatus.
An advantage of a communication system of the present invention is that time overhead for transmitting and receiving preamble and frame synchronization patterns is eliminated.
Another advantage of a communication system of the present invention is that a symbol clock recovery circuit and a frame detection circuit are not required for receiving a communication signal.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.